1. Field of the Invention
This invention relates to current transducers.
2. Description of Related Art
Current transformers (CT's) are a form of transducer by which a measure of a current in a conductor can be derived. They are electrically isolated from the conductor itself and, for this reason, they have found extensive use in fault protection systems and in the field of power electronics as, for example, monitors for current regulation.
A known CT relies on the substantial balance of magnetomotive forces (MMF) between primary and secondary windings that will exist in a CT using a high permeability core. FIG. 1 shows a simple CT with primary current I.sub.p secondary current I.sub.s, load or `burden` resistance R.sub.L, secondary winding resistance r.sub.s and a core of reluctance . A low secondary circuit impedance (`burden`) will mean that the balanced condition will be achieved at relatively low levels of core flux. This means that relatively little MMF is required to support the core flux, and that, therefore, the net ampere-turn product, taking both the primary and secondary circuits into account, will be close to zero. This means that: EQU n.sub.p I.sub.p +n.sub.s I.sub.s =0 (approximately)
and therefore: ##EQU1##
In practice, n.sub.p will usually be small and the primary may often comprise one or more turns of an external conductor passed through a circular or square core upon which many turns of secondary are wound.
Current transformers of this type are well-known and understood, and are in common use for alternating current measurement. They are discussed in, e.g., the book `Electrical Machines` by Slemon & Straughan, Addison Wesley, USA, 1980, which is incorporated herein by reference.
Frequency-domain analysis of the simple CT model shown in FIG. 1 shows that the frequency response is as shown in FIG. 2 when I.sub.p is supplied from a sinusoidal alternating current source. As can be seen, the response is accurate only where the frequency is well above the "cut-off" value .omega..sub.c. Below .omega..sub.c, the response falls with decreasing frequency, and I.sub.s. will ultimately be zero where I.sub.p is of zero frequency--i.e. there will be zero response to a constant, direct, current.
The fall in response for frequencies below .omega..sub.c may be understood by considering the core flux, which (given r.sub.s +R.sub.L is non-zero) will be proportional to the integral of the secondary voltage. In the case of an alternating waveform the amplitude of the core flux will be inversely proportional to the frequency of the monitored current. In addition, the finite permeability of a real core requires MMF to drive the flux around the core. Assuming a linear response of the magnetic material of the core, this MMF will be directly proportional to the flux. As the core flux increases a larger MMF will be needed to support it. Thus, with decreasing frequency the CT core absorbs an increasing proportion of the primary MMF. Therefore, the secondary MMF and the output current must fall.
It has been considered that a tail-off in the lower frequency response of a CT presents an operating limit on its usefulness. A low frequency CT needs both a large core and a low secondary impedance to offer a flat frequency response over a specified working frequency range. In the limit, known CTs cannot operate at dc (zero frequency) because of the non-zero secondary circuit resistance and non-zero core reluctance which are present in practice.
GB 2034487A, which is incorporated herein by reference, discloses a current transformer where the secondary winding is connected to an operational amplifier configured as an integrator. Although this system allows some degree of compensation for changes in temperature of the current transformer, it still cannot extend the frequency response down to zero frequency.
To address the problem of measuring currents at low frequencies and at dc, current measuring devices have been developed that rely on the Hall-effect. These are responsive to the strength of the magnetic field created by the current to be monitored. They are also often referred to in the art as current transformers' although conventional transformer principles are not involved.
A known current transducer based on the Hall-effect uses a Hall-effect device arranged in a gap of a toroidal core. The conductor carrying the current to be monitored is arranged to pass through the toroid. The Hall-effect element measures directly the flux resulting from the introduction of MMF in the toroidal core due to the current in the conductor.
While the device is relatively simply constructed, it has some disadvantages. Firstly, the response of the core material is not linear, which results in a non-linear relationship between primary current and core flux, and hence an error in the output. Secondly, the Hall-effect device itself also has a non-linear response and displays dc offset characteristics which will introduce error into measurements. Furthermore, the small amplitude of the Hall voltage output requires relatively large gain amplification which may render the transducer unacceptably prone to noise.
Feedback has been used in conjunction with a CT and a Hall-effect element. In this arrangement the drawbacks associated with a conventional CT are addressed by actively driving the secondary current from an amplifier having an input which is a negative feedback signal from the Hall-effect element proportional to core flux. The secondary MMF is then independent of burden voltage and can be made to follow the MMF due to the current in the primary conductor as closely as necessary by adjusting the product of the gain of the feedback amplifier and core permeability. With very large amplifier gain (and high core permeability) the balance between the primary and secondary MMFs is determined only by the offset null of the Hall-effect device. Core linearity becomes largely irrelevant because the feedback action is always such as to maintain zero flux. The ratio of primary to secondary current is, therefore, determined by the ratio of primary to secondary turns only.
Such current transducers of the `flux nulling` Hall-effect type have become popular in the electric machine control field (for example for switched reluctance motors and generators) because of their dc response, wide bandwidth and small size. An example of the flux-nulling sensor is one manufactured by LEM s.a. of Geneva, Switzerland. These sensors are non-invasive and electrically isolated from the monitored current. However, they are relatively expensive because they need an accurately zeroed Hall-effect element and fast responding amplifiers. Furthermore, the core can still take up a significant amount of space.